Meiosis and sporulation of the yeast Saccharomyces cerevisiae can be initiated by partial deprivation of carbon, nitrogen, phosphorus, or sulphur sources. It can also be induced by the deprivation of guanine or, less efficiently, uracil nucleotides. It can be prevented by the addition of methionine plus adenine or by S-adenosyl-methionine (SAM). The results suggest that meiosis may be controlled by some methylation reaction.